In U.S. Pat. No. 5,463,565, entitled "Data Block Format for Software Carrier and Player Therefor," granted on Oct. 31, 1995 in the names of Cookson and Ostrover, which patent is hereby incorporated by reference and hereinafter referred to as the "'565 patent," there is disclosed an optical disk player that is capable of playing either of two versions of the same motion picture. The motion picture is represented on the disk as "compressed video" in the form of successive data blocks in a single track, with each block possibly containing video information, audio information, subtitle information, and the like. The '565 patent explains how a huge quantity of data can be represented on a disk no larger than the present-day audio CD. Not only is it possible to include sound tracks in multiple languages, but it is even possible to store two versions of the same motion picture, e.g., R-rated and PG-rated.
Two versions of the same motion picture usually contain a great deal of common material. This is especially true of R-rated and PG-rated versions of the same program, but the same thing applies to other examples of multiple versions (at least two) of the same program material. For example, the same disk might include teaching and testing materials, with common subject matter being included in both "versions." To minimize redundant storage of data, three types of data blocks are provided in the same track, in an interleaved fashion--blocks which contain material unique to one version, blocks which contain material unique to the other, and blocks which contain material common to both. A series of codes and pointers included in each block allow play of common blocks when either version is being played, and play of blocks of only one of the two other types depending on which version has been selected; blocks which contain data unique to the unselected version are skipped over.
When more than one version of a program is to be derived from a digital optical disk system, there may be instances where mechanical and buffer management limitations would otherwise prevent seamless derivation of the alternate versions. The term "seamless play" refers to sufficient data always being available in the necessary buffers to allow continuous play, with no apparent interruptions, even though the player read head has to skip over unselected data blocks. While the player read head is skipping over an unselected sequence of data blocks, data for the version being played is furnished by buffers which were replenished prior to the start of the jump. But if a sequence of data blocks that must be skipped is very long, then the read head may not be able to reposition itself in time to reacquire data for a particular buffer (video, audio, etc.) which has been emptied during the jump. In such a case, by the time the read head repositions itself and starts to replenish the empty buffer(s), the screen may have gone blank or the soundtrack may have stopped.
The larger the buffers, of course, the greater the amount of data that can be stored in them, and the longer the allowable time for a jump without the buffers becoming depleted. But there are practical limits to the sizes of the buffers. For example, consider a motion picture where, after common material, the play branches to either a ten-minute segment for one version or a twelve-minute segment for another, following which both versions would include some more common material. Of four successive block sequences stored on the disk, the first and last might represent common material, and the two middle sequences might represent respective versions. During play of the disk, the read head must skip over a block sequence representing either ten minutes or twelve minutes. Such a large jump may require a longer time than one or more of the buffers can sustain. In such a case, the play will not be seamless. It is not practical to avoid this problem by saying that the buffers should be larger because the additional memory may add significantly to the cost of the player and, within practical limits, there will always be some multiple version disks which just cannot be played seamlessly.